We propose to study the potential role of anatomical reorganization in the production of anomalous pain syndromes associated with spinal cord lesions. Baseline data will be gathered regarding the normal cytology and synaptology of the primate dorsal horn, with special attention directed towards delineating the distribution and character of presynaptic elements arising from primary afferent neurons. An attempt will be made, using horseradish peroxidase labelling techniques, and dorsal rhizotomies, to provide ultrastructural criteria for the identification of small caliber fibers thought to transmit noxious and thermal stimuli, and large caliber fibers subserving tactile and proprioceptive input. Additionally, the nature of pre-synaptic elements arising in brainstem systems known to modulate the processing of nociceptive information in the dorsal horn will be assessed. These baseline data will provide the background for an analysis of synaptic reorganization consequent to spinal hemisection. This analysis will focus on the potential redistribution of primary afferent inputs via the mechanism of collateral axonal sprouting of such fibers to post-synaptic neuronal elements partially denervated by the lesion. Parallel behavioral studies will examine post-operative changes in the elicitability and magnitude of flexor withdrawal reflexes, providing functional measures of the activity of spinal cord nociceptive mechanisms for correlation with observed anatomical changes.